My first down bag started life as -5degC (26 yrs ago), but it never survived its one and only incredibly careful wash following every minute instruction.

The main problem is cold spots where i can't keep enough down staying in place. It's still very warm where the down is. I thought if i could get all the down on top / sides, it would increase the warmth a lot. It's a mummy shape with an extra zip in the foot box.
Maybe i don't need to do anything except figure out how to keep it from falling off as i toss and turn, and to keep it around me, not ‘waste’ down that’s lying beside my mat.
I toyed with trying my largest silk inner sheet as an outer ‘cocoon’. Still plenty of room for the down to loft well, but it keeps the bag where i need it. Trouble is, it’s far too hot to actually try out at home more than for a few minutes! Not so good for keeping the bag clean, but this bag doesn’t owe me anything after years of use.
Is there a simple, light weight way to ‘attach it’ under the mat? I mean something like 3 elastic ‘bands’ sewn on both sides of the bag, which slip under the mat. But i thought it might still allow cold air in between the bands of elastic, and too much elastic will defeat the lightweight purpose. I thought if I could shake most of the down away from the edges and stitch lengthwise to stop it moving back, those empty shell sections could wrap around the edges of my mat.
Any comments?